1. Field of the Invention
The present invention relates generally to a laser Doppler velocimeter of the type used to measure a localized fluid velocity and, more particularly, to a laser Doppler velocimeter employing a reflector in the forward direction which reflects scattered radiation in an antiparallel direction back to a collector proximate a laser source.
2. Description of the Prior Art
Presently, laser Doppler velocimeters (LDV's) are used to optically measure the velocity of flow of a fluid. Such velocimeters can provide a dynamic measurement of flow velocity at a location within the fluid without requiring a probe or other structure to be disposed in, and hence disturbing, the flow. Basically, a laser velocimeter provides a velocity measurement of a fluid flowing along a path by focusing one or more coherent beams of laser light onto a point within a fluid stream containing particles having diameters of the order of the laser light wavelength. The movement of the flowing particles serves to "scatter" the light radiation by the Mie scattering process, and shift its wavelength by an amount dependent upon its velocity. Hence its wavelength is shifted by an amount dependent upon the velocity of the fluid. The described Doppler shifting of the wavelength is measured by various techniques to provide a measurement of the velocity of the particles.
One type of laser Doppler velocimeter is commonly referred to as the "dual scatter" type which provides a relatively high signal-to-noise ratio and a relatively high sensitivity compared to other instruments for measuring fluid velocity. Such a velocimeter generally includes a laser source providing a pair of parallel coherent beams which are focused to a crossing point at a location at which it is desired to measure the velocity of the fluid flow. A fringe field is formed at the beam crossing location due to beam interference in a manner well known to the art. As a particle within the fluid moves through the fringe field, the light intensity scattered therefrom is modulated at a frequency which is proportional to the scalar component of the velocity of such particle in a direction which lies in a plane normal to the bisector of the angle formed between the pair of beams at their point of incidence on the particle. Such Doppler shifted scattered light radiation is collected by a photo detector which provides a signal indicative of such velocity component.
A particular kind of dual-scatter velocimeter is the so-called confocal back-scatter on-axis velocimeter which uses a common lens for both the transmission of the output beams and collection of the back-scatter Doppler shifted radiation.
Another kind of dual-scatter velocimeter is one which includes a local oscillator which is capable of simultaneously generating signals representative of two components of the flow velocity in directions transverse to and along the transmitting axis of the system. In such a velocimeter a comparator is included for comparing the Doppler shifted wavelength of the radiation scattered by the particle from one of the two transmitted beams with the wavelength of such beam prior to the Doppler shift and for providing a signal indicative of a velocity component of the particle along the transmitting axis. This enables two separate scalar velocity components along known paths to be obtained with one velocimeter from which the path and speed of the particle in two-dimensional flow is able to be calculated.
Heretofore, the described LDV systems have performed satisfactorily in environments where the fluids have included a substantial number of particles so that backscatter techniques can be used and where the collector is able to be placed in a direction generally forward of the fringe field such that it collects the relatively strong forward scattered light. However, in the field of aeronautics where it is desired to measure the velocity of air at or near a given airfoil such LDV's have been unsatisfactory in view of the low backscatter obtained in air and in view of the problems encountered in maintaining alignment between a collector and a transmitter when the object carrying the transmitter is moving.
For example, in an attempt to increase the concentration of particles in air so that the velocity of the air could be measured by backscatter techniques, a smoke screen is laid by an aircraft traveling in one direction to enhance the backscatter properties of the air. Such aircraft is then flown back through the smoke screen. This technique is not only operationally inconvenient but creates a turbulent environment which is dangerous for the aircraft and which disturbs the velocity to be measured.
Similar problems are encountered in wind tunnel applications in that the more convenient backscatter LDV systems generally provide inadequate signals while the forward scatter LDV systems require that alignment be maintained between the moving collector and the transmitter. It should also be noted that in some wind tunnel applications, the presence of large objects in the tunnel between the transmitter and the collector preclude the use of forward scatter systems.
Examples of prior art laser Doppler velocimeters are found in U.S. Pat. No. 3,519,356, "Ring Laser Flow Meter with Means to Compensate for Changes of Refractive Index of the Flowing Medium," Kroeger et al; U.S. Pat. No. 3,822,940, "Velocimeter", Goldfischer; U.S. Pat. No. 3,856,402, "Clear Air Turbulence Detector," Low et al; and U.S. Pat. No. 3,915,572, "Combined Dual Scatter Local Oscillator Laser Doppler Velocimeter," Orloff. However, none of these patents reveal a laser Doppler velocimeter having an antiparallel reflector in the forward direction.